This invention relates generally to optical communication networks.
Optical communication networks may be subject to chromatic dispersion, which is the dependence of the refractive index of a transmission medium on the wavelength of light traveling through the medium. Thus, dispersion corresponds to a change of the light velocity in a waveguide or a medium, depending on its wavelength. An optical network's dispersion dependence on wavelength results in pulse spread. Dispersion restricts the information carrying capacity of a waveguide since, the wider the pulse, the fewer pulses that can be accommodated per interval, resulting in a smaller bit rate.
There are a number of different ways to compensate for chromatic dispersion. A long length of specialty optical fiber with a fixed negative dispersion coefficient can be used. This method may provide good compensation, but it is fairly lossy and relatively bulky, since the fiber spool may be from 10 to 20 kilometers. Also, the use of the specialty optical fiber provides no tunability to the dispersion compensation.
Another approach is to use a chirped fiber Bragg grating. A fiber Bragg grating includes a structure whose refractive index periodically changes in value as a function of position in the structure. A small portion of the overall light may be reflected at each refractive index change. For a wavelength of light satisfying the Bragg condition, the refracted portions interfere constructively to produce high reflection.
In a chirped fiber Bragg grating, the optical grating period changes linearly over the length of the grating. Thus, a chirped fiber Bragg grating reflects a set of wavelengths. An input pulse may be directed to a chirped fiber Bragg grating. The shorter wavelengths are reflected sooner in the grating, while the longer wavelengths penetrate deeper into the grating before reflecting. Thus, the shorter wavelengths have less delay than the longer wavelengths and this exactly compensates for the delay introduced by previous propagation in a fiber.
However, the use of chirped fiber Bragg gratings is also disadvantageous in some respects. Generally to introduce a chromatic dispersion of, for example 850 ps/nm, the grating length will depend on the device bandwidth and, for the C band, which is approximately 30 nanometers, the required length is approximately 2.6 meters. This length of fiber Bragg grating is difficult to fabricate with high quality and it is difficult to package. Furthermore, channel dispersion cannot be individually tuned, for example to compensate the second or third order dispersion of the transmission fiber.
Thus, there is a need for an improved dispersion compensator.